This invention relates to an optical sensor having a light transmitter for transmitting pulsed light and a light receiver for receiving light and determining whether the received light was from the light transmitter, as well as a method of suppressing effects of interference light on the light received. More particularly, this invention relates to such an optical sensor and such a method wherein the pulsed light is transmitted according to a specified pattern including a pair of pulse packages separated by a specified time interval and each including a plurality of pulses following one another at a specified carrier frequency.
International patent publication WO 96/38742 disclosed a device for eliminating interference (noise) signals in a light barrier, comprising a comparator for filtering out noise signals with small amplitudes. A transmitter transmits periodic light pulses with a specified frequency and the same frequency is also set for the light receiver. The light receiver is activated in synchronism with the transmitted light signal. A period of pause after each activation period is adjusted to be equal to the interval between a pair of successive pulses transmitted.
German patent publication DE 42 24 784 C2 disclosed a method of eliminating the effects of noise light in a light barrier. The groups of coded light pulses are transmitted through a time window which is smaller than an expected minimum interval between noise pulses (say, from a fluorescent lamp). Even with a noise pulse superposed, a noise-free pulse group will always be received by the receiver, and it is sufficient to conclude that the transmitted light has been received.
German patent publication DE 31 19 876 A1 disclosed an infra-red transmission/reception system, say, for a light barrier arrangement, providing for multiple coding of transmitted signals, for example, by modulating a pulse-coded signal with a carrier frequency. Multiple coding permits several light barrier arrangements to be operated concurrently if each arrangement uses a different coding. The carrier frequency used was 31.25 kHz.
Modem fluorescent lamps generate modulated light with high frequencies with basic frequency of about 100 kHz in the case of a so-called energy-saving type. With a linearly elongated type of fluorescent tube, light from the center portion of the tube is modulated with basic frequency of 60-80 kHz and it is about 30-40 kHz at the end parts of the tube. Steep edges entail high harmonic components due to which even higher frequencies are generated.
With increasing popularity of fluorescent lamps, there is an increased problem of interference on conventional optical sensors. For optical sensors with a light transmitter and a light receiver which are separated, or light barriers, suppression of noise (interference) light is a critically important problem.